All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, scorzonera, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of pests and diseases such as downy mildew (Bremia lactucae). Every year this disease leads to millions of dollars of lost lettuce crop throughout the world. Downy mildew (Bremia lactucae) is highly destructive on lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae, which can be one of the following strains: NL1, NL2, NL4, NL5, NL6, NL7, NL10, NL12, NL13, NL14, NL15, NL16, Bl:17, Bl:18, Bl:20, Bl:21, Bl:22, Bl:23, Bl:24, Bl:25, Bl:26, Bl:27, Bl:28, Bl:29, Bl:30, Bl:31, Bl:32 (Van Ettekoven, K. et al., “Identification and denomination of ‘new’ races of Bremia lactucae,” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175; Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants); IBEB press release “New race of Bremia lactucae Bl:27 identified and nominated”, May 2010; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), “New race of Bremia lactucae Bl:28 identified and nominated”, March 2011; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), IBEB press release, “New races of Bremia lactucae, Bl:29, Bl:30 and Bl:31 identified and nominated”, August 2013; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), IBEB press release, “A new race of Bremia lactucae, Bl:32 identified and nominated in Europe”, May 2015), Plantum NL, (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), IBEB-EU press release, “A new race of Bremia lactucae, Bl:33EU identified and denominated in Europe”, May 2017), Ca-I, Ca-IIA, Ca-IIB, Ca-III, Ca-IV (Schettini, T. M., Legg, E. J., Michelmore, R. W., 1991. Insensitivity to metalaxyl in California populations of Bremia lactucae and resistance of California lettuce cultivars to downy mildew. Phytopathology 81(1). p. 64-70), and Ca-V, Ca-VI, Ca-VII, Ca-VIII (Michelmore R. & Ochoa. O. “Breeding Crisphead Lettuce.”In: California Lettuce Research Board, Annual Report 2005-2006, 2006, Salinas, Calif., pp. 55-68).
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribisnigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers.
The head size of iceberg lettuce is a measure of yield. This is especially true in the production of iceberg lettuce heads to be used in processing, where yield is a very important trait. An iceberg lettuce variety used in processing should have at least a medium to large head size. Medium-sized iceberg lettuce heads are required in the market, due their optimum combination of yield and suitability for the average household size. Too large heads, weighing up to two pounds per head, take too long to consume for an average American household size, which has decreased until 2.6 people per home. This household eats on average 0.12 pounds of iceberg lettuce per day. On the other hand, small iceberg lettuce heads provide the grower insufficient yield per hectare to make up for the growing costs. Therefore medium-sized iceberg lettuce varieties are considered the ideal compromise between consumer's satisfaction and grower's yield.
Vegetables are generally considered healthy foods. Both consumers as well as producers benefit from an increase of vegetable consumption. Higher vegetable consumption can be promoted by education, marketing, and improvement of certain product traits, such as taste, aroma, and visual attractiveness.
Iceberg lettuce is also indicated as crisphead lettuce, although crisphead also involves the Batavia subtype. It is part of Lactuca sativa L. (Capitata Group). Iceberg lettuce is a subtype of cultivated lettuce with large firm heads, giving only slightly to pressure when mature (Ryder E. J., 1998, Lettuce endive, and chicory. CABI publishing, Wallingford, Oxon, UK).
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.